1. Field of the invention
The present invention relates to a toner for forming an image by developing an electrostatic image, as in electrophotography, electrostatic recording or electrostatic printing. It also relates to a toner image fixing method and an image forming apparatus that make use of the toner, and a resin composition.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are conventionally known for electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure or solvent vapor.
Various methods or techniques have been developed in relation to the above final step, i.e., the step of fixing the toner image to a sheet such as paper. A method most commonly available at present is the pressure heating system making use of a heating roller.
The pressure heating system making use of a heating roller is a method of carrying out fixing by causing an image-receiving sheet to pass over a heating roller whose surface is formed of a material having a releasability to toner while a toner image surface of the former is brought into contact with the surface of the latter under application of a pressure. Since in this method the surface of the heating roller comes into contact with the toner image of the image-receiving sheet under application of a pressure, a very good thermal efficiency can be achieved when the toner image is melt-adhered onto the image-receiving sheet, so that fixing can be carried out rapidly. This method is therefore very effective in high-speed electrophotographic copying machines. In this method, however, since the surface of the heating roller comes into contact with the toner image under application of a pressure, part of the toner image may sometimes adhere and transfer to the surface of the fixing roller, which may re-transfer to the subsequent image-receiving sheet to cause an offset phenomenon, resulting in a contamination of the image-receiving sheet. Thus, it is essential in the heating roller fixing system that no toner is adhered to the surface of the heat fixing roller.
Accordingly, under existing circumstances, it is sought to provide a binder resin for toner, having a broad fixing temperature range and high anti-offset properties.
Research has been conducted regarding two-color copying machines or full-color copying machines, and many of which have been put into practical use. For example, Journal of Electrophotographic Society, Vol. 22, No. 1 (1983) and Journal of Electrophotographic Society, Vol. 25, No. 1, p. 52 (1986) make reports on color reproduction and gradation reproduction.
However, images formed by full-color electrophotography presently put into practical use are not necessarily satisfactory to those who are accustomed to seeing color images that are not directly compared with actual things or objects as in television pictures, photographs and color gravures or that are made artificially more beautiful than actual things.
In the formation of color images by full-color electrophotography, all colors are reproduced usually using three-color toners of yellow, magenta and cyan corresponding to the three primary colors, or four-color toners in which a black toner is used in addition to the three-color toners. In a method commonly used therefor, first, light reflecting from an original is passed through a filter capable of transmitting color-separated light that stands in a relation of a complementary color with respect to the color of a toner, and an electrostatic latent image is formed on a photoconductive layer. Next, a toner image is formed on a support (an image-receiving medium) through development and transfer steps. Subsequently, the above procedure is successively repeated plural times so that the subsequent toner images are superposed on one another on the same support while in proper registration, and thus a final full-color image is obtained through a fixing step.
In the full-color electrophotography, development is carried out plural times using plural kinds of toners with different colors, and toner layers are superposed on the same support to form a full-color image. For this reason, binder resins used for color toners are required to satisfy conditions as itemized below.
(1) Fixed toner must be brought into an almost perfectly molten state such that the form of toner particles can not be distinguished, in order that the fixed toner not cause irregular reflection of light to hinder color reproduction. PA1 (2) Since the toner layers are superposed, the binder resin must be transparent so that a different color tone present on a lower toner layer has is not affected. PA1 a latent image bearing member capable of bearing an electrostatic latent image; PA1 a charging means for statically charging said latent image bearing member; PA1 a latent image forming means for forming an electrostatic latent image on the latent image bearing member having been statically charged; PA1 a developing means for developing said electrostatic latent image to form a toner image on said latent image bearing member; PA1 a transfer means for transferring said toner image to a transfer medium from said latent image bearing member; PA1 a cleaning means for cleaning the surface of said latent image bearing member to remove toner remaining thereon untransferred; and PA1 a fixing means for fixing the toner image transferred to said transfer medium by the action of heat and pressure; wherein; PA1 said developing means retains a toner comprising toner particles containing i) a binder resin comprising a resin composition with a domain-matrix structure and ii) a colorant: PA1 said resin composition with a domain matrix structure being comprised of a resin P1 that forms domain particles and a resin P2 that forms a matrix: said resin P1 having a glass transition temperature Tg1 of from 0.degree. C. to 60.degree. C., and said resin P2 having a glass transition temperature Tg2 of from 40.degree. C. to 90.degree. C., provided that the glass transition temperature Tg2 of said resin P2 is at least 5.degree. C. higher than the glass transition temperature Tg1 of said resin P1; and said domain particles having an average particle diameter of not larger than 5 .mu.m.
As stated above, when used in monochrome copying machines, binder resins used for toner are required to give a broad fixing temperature range and high anti-offset properties. When used in full-color copying machines, binder resins are required not only to have a broad fixing temperature range but also to be transparent and to give a flat image surface when images are fixed.
The transparency of resin and flatness of fixed-image surface as stated above have a great influence on image quality not only in the case when the toner image is fixed on a non-light-transmissive transfer medium such as paper and the image reflected therefrom is viewed but also in the case when the toner image is fixed On a light-transmissive transfer medium such as an OHP sheet and the transmitted light image is viewed on a screen.
Moreover, in recent years, in the field ranging from monochrome copying machines to full-color copying machines, a variety of features is required, e.g., to take copies at higher speed, to shorten the heat-up time and to decrease power consumption.
In order to satisfy these requirements, it is necessary to provide a binder resin for a toner that enables low-temperature fixing, and also, which as stated above, can give a broad fixing temperature range, has an excellent transparency, and can give a flat image surface when images are fixed.
Meanwhile, one may contemplate a method making use of pressure fixing toners. In this method, the binder resin can not melt when the toners are used as toners for full colors in which three or four colors are superposed to effect color reproduction, so that color-mixing performance becomes poor to give a dull, chroma-poor image. Hence, in the fixing step, a heat must be applied to the extent the binder resin can melt and achieve color mixture.
Only for the purpose of achieving low-temperature fixing, it is possible to decrease melt viscosity of binder resins for the toner. For example, there is a method in which the molecular weight of resin or glass transition point thereof is lowered. This method, however, may result in a poor storage stability of toner to tend to cause phenomena such as blocking between toners and melt-adhesion of toner to a developing drum.
Hitherto, for the purpose of expanding the fixing temperature range of vinyl polymers, methods in which an anti-offset agent is used are disclosed in Japanese Patent Applications Laid-open No. 58-14148, No. 58-72948, No. 59-174855, No. 59-174856 and No. 60-123855, and Japanese Patent Publications No. 52-3304, No. 52-3305, No. 57-52574 and No. 58-8505. These, however, are supplementary means, and may damage the transparency of toner when applied in monochrome toners or may bring about a poor color mixing performance when applied in full-color toners.
Japanese Patent Applications Laid-open No. 56-158340, No. 58-86558, No. 58-203453, No. 59-88748, No. 59-226358, No. 60-45259, No. 60-45261 and No. 60-46566 and Japanese Patent Publication No. 60-2411 disclose binder resins for toner that have a low-molecular weight component and a high-molecular weight component. Use of such resins has made it possible to expand the fixing temperature range to a certain extent, but on the other hand causes the problem that grindability is lowered or melt viscosity becomes excessively high at the time of heat kneading, because of the presence of high-molecular weight components such as gels. Particularly when such binders are used in full-color toners, there is the problem that the smoothness of image surfaces when images are fixed is damaged, resulting in a poor color mixing performance.
U.S. Pat. No. 4,925,765 discloses a negative solid block toner wherein an AB type, BAB type or ABA type block copolymer is used as a charge control agent.
In this block copolymer, a copolymer comprising acrylic monomers or a copolymer comprising methacrylic monomers is used as segment-A and a copolymer comprising monomers selected from the group consisting of styrene, a substituted styrene, butadiene, and an acrylate and/or a methacrylate as segment-B.
It can be presumed that use of such a block copolymer as a binder resin for toner causes the problems that the grindability in the preparation of toner becomes poor to make particle size distribution broad and also the environmental stability of toner becomes poor, because of the segment-A which is an acrylic copolymer or methacrylic copolymer.
Thus, it is very difficult to satisfy at the same time carrying out fixing at a low temperature, expanding the fixing temperature range and ensuring the properties of toner, i.e., storage stability, fluidity, durability, transparency, and smoothness of fixed-image surface.
As for fixing rollers, they can be roughly grouped into a silicone rubber roller and a fluorine type roller.
When the silicone rubber roller is used as a fixing roller, a high-temperature offset tends to occur as a result of its repeated use regardless of whether or not a release oil has been applied. In the case of the silicone rubber roller, the release properties can be maintained to a certain extent since the smoothness or cleanness of the roller surface is not damaged at the initial stage where its use is started. However, repetition of full-color copying in which, as in the case of full-color images, image areas are larger and toners are held on the support in a much greater quantity than in the case of black and white images may result in a gradual lowering of the release properties of the roller surface. The degree of this lowering of release properties is several times that in black and white copying. This may cause what is called a high-temperature offset, which is a phenomenon in which a coating of toner or granular deposits are formed on the roller surface after full-color copies are made for only several thousand to several ten thousand sheets or an upper layer portion of an image surface is torn off when a full-color image is passed through a heat roller.
The fluorine type roller has in general a good durability, but tends to undergo a stretching because of pressure, thus having disadvantages such that it causes a lowering of resolution in copied images and causes conspicuous background staining. In order to eliminate such disadvantages, there is a disclosure (Japanese Patent Publication No. 58-43740) of a roller comprising a rubber covered thereon with a PFA (perfluoroalkoxyl resin) tube of 300 to 100 .mu.m thick. Use of such a roller can better prevent the resolution of copied images from being lowered because of the spread of toner under pressure.
In general, however, when the fluorine type roller is used as a fixing roller, a pressure roller is used which comprises a mandrel whose periphery is covered with a relatively thick, elastic material layer of rubber or the like, as disclosed by the present applicant in Japanese Patent Application Laid-open No. 61-89845.
In such an instance, as shown in FIG. 5, the paper output from a fixing roller after fixing of an image is in the direction inclined toward the fixing roller side with respect to the direction perpendicular to a line connecting the centers of a fixing roller 11 and a pressure roller 12.
Hence, the fixed image, even after it has passed the nip portion at which the fixing roller and pressure roller come into contact with each other is pulled along the fixing roller to cause what is called a "winding" phenomenon, so that the offset occurs. In order to prevent this phenomenon, a method is available in which a separation claw for paper output is provided. This separation claw, however, is in contact with the fixing roller, and hence may scratch the roller or may make a streak on the image surface, resulting in a serious lowering of image quality particularly in the full-color copying of photographs with wide image areas.
To solve or make small this problem, various measures have been attempted in fixing devices and toners, but no satisfactory solution can be said to have been achieved.
In the fixing devices or apparatus, materials with excellent surface release properties are used. It has been also known to apply an oil to the roller. In almost all heat-roller fixing devices presently put on the market, oil is applied anyway. However, application of oil in a large quantity for the purpose of improving release properties has caused undesirable problems such as oil contamination of copy sheets and an increase in cost.
From an approach from the toners, a method is taken in which a polyethylene or wax with a molecular weight low enough to be well melted when heated is added so that the release properties can be improved. This method can be effective for preventing offset, but on the other hand its addition in a large quantity may bring about undesired influences such that the transparency of OHP images of full-color toner is damaged, the static charge characteristics become unstable and the durability is lowered. Thus it is hard to say that this method is satisfactory.
In particular, as a problem peculiar to the full-color copying, the colors of toners of at least three colors, preferably four colors must be harmoneously balanced, and hence all the colors must be well balanced in respect of fixing performance or color reproducibility.
In theory, almost all colors could be reproduced by the subtractive color process if there are the three primary colors, the yellow, magenta and cyan colors. For this reason, presently commercially available full-color copying machines are so constituted that color toner images of the three primary colors are superposingly used. This ideally should enable achievement of the reproduction of all colors in all density ranges, but, in practice, more improvements must be made in some ways such as spectral reflection properties of toners, color mixture performance required when toner images are superposingly fixed, and prevention of a lowering of chroma.
When a black color is obtained by superposing the three colors, it follows that, as previously stated, toner layers three times thicker than a monochrome layer are formed on a transfer sheet, bringing about more difficulties in respect of anti-offset properties.